1. Field of the Invention
The present invention is directed to a contact-free, magnetic detection method and means for measuring physical conditions, such as stress and temperature, affecting an object. Any physical condition which affects a change in the magnetic properties of ferromagnetic materials can be measured.
2. Description of the Prior Art
It is well known that the magnetization of a ferromagnetic material changes in discrete pulses, rather than continuously, as a function of an applied magnetic field. This phenomenon is known as a Barkhausen effect (or small Barkhausen effect), and it is evidenced by small clicks (i.e., Barkhausen noise) when the magnetic domains of a ferromagnetic material are switched as a result of an applied magnetic field. The small Barkhausen effect may be amplified if the domains of the magnetic material are predominantly aligned before the magnetic field is applied. When the domains are predominantly aligned there is an abrupt switching in the magnetization of the magnetic material at a certain threshold magnetic field. This is known as a large Barkhausen effect. An example of the large Barkhausen effect is described, for example, in an article entitled "Weigand Wire: New Material for Magnetic-Devices", which appeared in ELECTRONICS MAGAZINE, July 10, 1975, on pages 100-105.
Methods utilizing both the large Barkhausen effect and the small Barkhausen effect have been developed. For example, U.S. Pat. No. 3,427,872 issued to R. W. Leep and R. L. Pasley employs a first coil and/or magneto-coil element to induce magnetization into the test specimen and uses a second coil means for detecting Barkhausen noise (i.e. a small Barkhausen effect) as the magnetization in the specimen is varied. Problems arise with devices such as the Leep device which detect Barkhausen noise because the level of signal obtainable from these devices is too faint and the response time is too slow to give meaningful analysis of stress and/or torque in rapidly rotating shafts or the like.
U.S. Pat. No. 4,416,161 issued to S. Barkhoudarian discloses a torque-measuring apparatus which utilizes a Wiegand device mounted on a rotating shaft. A large Barkhausen effect is generated when a magnetic field is generated in the vicinity of the loaded shaft. The Barkhoudarian device determines the torque on the shaft by measurement of either the large Barkhausen effect pulse width or amplitude.
Inherent difficulties result from using the measurement of pulse width or amplitude to characterize the stress on the shaft because these parameters, in addition to varying as a function of torque, also vary as a function of the shaft speed. Hence, measurements using these parameters are excellent if the speed of the shaft remains constant, but are very difficult to make if the speed of the shaft changes during the test. The correction for speed is non-linear, making calculations of torque very difficult.